(1) Field of the Invention
The present invention relates to imaging systems and more specifically to the pixel structure in an imaging system.
(2) Description of the Related Art
Image sensing devices are generally characterized as the light detecting component in imaging systems. An image sensing device in a camera, for example, uses light to capture an image so as to replace traditional film-based systems. The image sensing device in a camera is configured to capture a monochrome or color image by way of charge coupled device (CCD) technology, or field effect transistor (FET), bipolar junction transistors (BJTs), diode devices, or photoresistors fabricated with complementary metal oxide semiconductor (CMOS) devices.
Imaging systems built with CCD or CMOS based image sensors generally require an infrared (IR) filter as part of the optical system. This need arises because the most common semiconductor-based image sensing devices respond not only to visible light (approximately 380 to 780 nanometers), but also to infrared light in the range of approximately 780 to 1100 nanometers. Without an IR filter, it is virtually impossible to obtain a high-quality color image due to the effect of infrared signals which distort the image. Monochrome imagers also require an IR filter to correctly preserve scene luminance.
Conventionally, an IR filter is incorporated into the imaging system and is part of the optical train (i.e., somewhere in the optical system apart from the image sensing device) either overlying the lens or interposed within other optical elements. The disadvantage of the inclusion of a separate IR filter is that it adds costs (IR filters typically cost around $ 0.50 to $ 1.00) and is an additional component to the total system count, i.e., piece-part count. Also, the inclusion of this additional component must be accommodated in the design of the imaging system, thereby adding additional complexity and costs as each IR filter must be mechanically assembled into the imaging system. Moreover, depending on where the IR filter is placed, the size of the imaging system would be changed further adding costs to the imaging system. The system software and signal processing must also be adjusted to accommodate for the color performance characteristics of the IR filter. Additionally, if the IR filter is placed, for example, in front of the lens, the IR filter is exposed to the environment and therefore presents the risk of damage by the environment, such as by moisture or scratching.
Yet another concern with the use of IR filters in imaging systems is that of signal loss. The insertion of IR filters diminishes the overall signal-to-noise ratio of the imaging system as these filters typically pass approximately only 80 to 90% of light in the visible spectrum. Thus, an imaging system with an IR filter, having a lower signal-to-noise ratio, will yield a lower quality image as opposed to an imaging system having a higher signal-to-noise ratio.
It is desirable to provide an imaging system that eliminates the effect of infrared light upon the image sensors and yet does not require the use of an IR filter, thereby eliminating the additional costs and complexities associated with adding an additional component to the overall system.
A pixel structure for providing sensor signals in response to incident light. The pixel structure includes light selective elements, the light selective elements having predetermined thicknesses to absorb only light having wavelengths corresponding to the visible region of the light spectrum.